Preheat oven to 350 degrees. Lightly spray an 8×8″ baking dish or a 9″ round baking dish, set aside.
In the bowl of a stand mixer, beat together butter, sugar and vanilla until light and fluffy, about 2 minutes. Add egg, then egg yolk, mixing until just combined.
In a small mixing bowl, whisk together flours, baking powder, salt, and cinnamon.
With mixing speed on low, alternate adding the flour mixture with the sour cream and buttermilk (this will help prevent the mixture from curdling) into the mixture of butter, sugar, vanilla and eggs. Mix until well combined and no flour pockets remain. The cake batter will be relatively thick.
Remove bowl from stand and spread the batter evenly into the prepared pan.
Layer the sliced peaches on top, making sure to completely cover the batter.
In a small mixing bowl combine all crumb topping ingredients. Using a pastry blender, cut the butter into fine pieces, creating tiny lumps in the mixture. Pour the mixture over the peaches, and spread evenly.
Place in oven and bake for 35-40 minutes or until a toothpick inserted into the center of the cake comes out clean (I baked mine in a round dish and it took about 38 minutes). The topping should be nice and brown and the center should look slightly moist. Remove from oven and allow to cool for 30-45 minutes before serving.